We propose to test the hypothesis that recurrent stenosis following carotid endartererctomy can be reduced by pre- and post-operative oral administration of platelet-inhibiting drugs. This prospective randomized placebo controlled trial is designed to be carried out at a single institution and limited to two senior vascular surgeons using the same procedures. Preliminary data on 258 patients at this institution who were followed both angiographically and noninvasively with B-mode imaging and pulsed Doppler spectrum analysis showed very good sensitivity and specificity for the non-invasive procedures, and documented recurrent stenosis in 15-24% of untreated operated patients during the first post-operative year. Assuming an estimated entry rate of 100-125 patients per year, a 20% frequency of recurrent stenosis and a 50% benefit of treatment, a 3 year recruitment time will be required to achieve treatment and control groups of approximately 155 patients each. Follow-up will be 2 years. Since sample size requirements limit the study to placebo and treatment groups, we have chosen to use a treatment regimen of aspirin 330 mg and dipyridamole 75 mg given in combination three times daily. This trial will therefore critically assess a commonly used, but unverified regimen that consists of the therapy most likely to be effective (based upon the effect of this treatment on platelet consumption in patients with atherosclerosis, dose response studies in baboon models of arterial thrombus formation, and positive clinical trials in a) saphenous vein bypass, and b) peripheral arterial occlusive disease showing efficacy of the aspirin/dipyridamole combination over aspirin alone).